The invention relates to interlayers used for the production of laminated glazing intended for fitting into motor vehicles. Such glazing usually consists of two glass sheets separated from each other by a plastic interlayer, such as a sheet of polyvinyl butyral (PVB).
More particularly, although the invention is not limited to such applications, it applies to side windows linked to xe2x80x9craising-loweringxe2x80x9d systems defining a sliding movement of the said windows in the doors of motor vehicles.
Usually, such side windows consist of a sheet of so-called xe2x80x9csafetyxe2x80x9d or xe2x80x9csecurityxe2x80x9d glass, that is to say glass meeting European Regulation No. 43. Such windows have weaknesses with regard to the actual security of the vehicle since it is possible to break such windows and get inside the vehicle, either to steal the vehicle or to remove articles or accessories from the passenger compartment.
For a number of years now, anti-theft systems have been developed either in the form of an audible alarm or which allow the vehicle to be immobilized. Although such systems may possibly prevent the vehicle from being stolen, they are often ineffective against a simple break-in, by smashing the window in order to steal articles left in the passenger compartment.
In order to respond to and prevent this type of break-in, it has already been proposed to use laminated glazing consisting, for example, of two glass sheets separated by a plastic interlayer, as mentioned above.
The use of such laminated glazing is advantageous for solving the security problem. This is because the presence of the plastic sheet enormously complicates the breaking of the glass; in the case of laminated glazing comprising two glass sheets separated from each other by a sheet of PVB, when one or both glass sheets are broken, the laminated structure remains in place, the broken glass sheets remaining fast to the interlayer.
However, in the event of an attempted break-in, during which an individual attempts using a tool to smash the laminated glazing by repeated blows, it may happen that the glazing becomes deformed such that the edges of the latter come free of the lateral jambs into which it is set. This is more particularly true in the case of opening windows, such as those which slide vertically in the doors of motor vehicles; in the closed position, the lateral parts and the top of the window are not fixed to the door frame but set in the guide rails and the top of the frame. A large and repeated compressive force using a blunt instrument, such as a club or baseball bat, may thus result in the deformation of the window, which bends and comes free of the door frame, at least on one of its sides. It then merely remains to push or pull the window in order to expose an opening into the interior of the vehicle.
To limit or retard the deformation of such a window, when one or both glass panes are broken, it has already been envisaged to use a rigid interlayer which has a high flexural strength and therefore prevents the window from bending. However, such interlayers have large thicknesses which make them incompatible with the applications in question or at the very least require very significant modification to the door frames. The inventors were thus tasked with the objective to produce an interlayer consisting of at least one soft plastic film, as opposed to the abovementioned rigid interlayers, for the purpose of producing laminated glazing that can be used as a motor-vehicle window in standard structures, the said interlayer increasing the resistance of the window to deformation, or in other words increasing its xe2x80x9cresidual rigidityxe2x80x9d.
The present invention relates to an interlayer for a laminated glazing, consisting of at least two sheets of polyvinyl butyral (PVB), a reinforcement being inserted between the said two sheets of PVB, the said reinforcement giving the interlayer a tensile strength such that the elastic energy (Etr) needed to deform the interlayer in tension is at least 80 J/m2, in all directions in the plane of the interlayer.
Preferably, this elastic energy Etr is greater than 120 J/2.
This elastic energy needed to deform the interlayer in tension, which illustrates the tensile strength of the interlayer, is expressed in the following way:
Etr="sgr"e2X/2Yxe2x80x83xe2x80x83(I) 
with "sgr"e: tensile yield strength of the reinforcement,
Y: Young""s modulus
x: cross section of the reinforcement per unit length.
The elastic energy Etr of a sheet of PVB having a standard thickness for producing laminated glazing intended for motor-vehicle applications is very small and virtually equal to 0 J/m2.
Unlike the techniques envisaged hitherto, the interlayer has an almost non-existent flexural strength, yet it makes it possible to limit the deformation of the laminated window broken when pressure is exerted on it, because of the increase in its tensile strength, or more precisely because of the higher value of the elastic energy needed to deform the interlayer in tension.
In fact, during their studies the inventors have been able to demonstrate that, as soon as breakage of the glass sheet occurs, at least two pieces or fragments of glass are formed in the region where the force is exerted. The boundary or line separating these two pieces becomes a site of flexure allowing the window to deform. However, as soon as this deformation occurs, the two pieces of glass bear against each other and this results, since the interlayer remains fast to the glass sheets, in the said interlayer being stressed in tension, at least in one direction, and not only in bending.
According to a first embodiment of the invention, the reinforcement inserted between the sheets of PVB consists of yarns, such as glass yarns or nylon yarns, arranged in at least two directions, for example in order to form a grid or mesh.
In the case of such a reinforcement, the parameter x in formula (I) for Etr is such that x=IIr2n:
with
r: radius of the yarn
n: number of yarns present per unit length.
According to another embodiment of the invention, the reinforcement inserted between the sheets of PVB consists of a polyester film, such as a polyethylene terephthalate (PET) film having a thickness of greater than 70 xcexcm.
According to this embodiment, the parameter x in formula (I) for Etr is equal to the thickness of the sheet or film of the reinforcement.
The use of a polyethylene terephthalate film has in particular the advantage of possessing a tensile strength which remains high over a wide temperature range, at least up to 50xc2x0 C.
Advantageously, the reinforcement consists of the combination of at least two polyester films. This embodiment makes it possible in particular to retain good developability of the reinforcement in the form of film, when it is desired to increase the thickness of the said film in order to further improve the tensile strength. This is because increasing the thickness of such a film increases its stiffness and makes its developability less certain, especially if the window has a significant sphericity. The association or combination of two films results substantially in the same reinforcement in terms of tensile strength as a film having a thickness equal to the sum of the thicknesses of the two films in question and each of the films still having a developability compatible with windows having possibly very pronounced sphericities.
Also preferably, in the case of two polyester films joined together, these are separated by a sheet of PVB which ensures good cohesion of the assembly.
The invention also proposes laminated glazing consisting of at least two glass sheets and of an interlayer as described above having a tensile strength such that the elastic energy needed to deform it in tension is at least 80 J/m2 in all directions in the plane of the said interlayer.
Such laminated glazing therefore comprises two glass sheets which form the outer surfaces of the glazing and between which the interlayer may consist of two sheets of PVB between which a reinforcement, for example a PET film, as inserted.
Advantageously, the glazing comprises two glass sheets, so that the glazing according to the invention has firstly a good resistance to deformation in the case, as described above, of a thrust force exerted by a blunt instrument, for example on the glass sheet forming the outer surface of the glazing fitted into the frame of an opening, and secondly such glazing also makes it possible to have a satisfactory resistance to deformation in the case of a tensile force exerted on the glazing, for example using an instrument which, after having passed through the glazing, bears on the inner glass sheet and can exert a tensile force resulting in deformation of the glazing. In the latter case, the inner glass sheet breaks and it is the pieces of glass resulting from this glass sheet which, by bearing on each other, transforms the force exerted on the glazing by the instrument into a tensile force which is exerted on the interlayer.
Laminated glazing comprising two glass sheets therefore provides a better defense against attempted break-ins since it makes it possible to withstand thrust or tensile forces exerted, the direction of the forces being the same, that is to say approximately perpendicular to the plane tangent to the glazing at the point where the said force is exerted.
According to embodiments of the invention, such laminated glazing is fitted into the sides of a motor vehicle and is mounted so as to be able to move. In fact, the need to offer resistance to deformation of the glazing, when the glass sheets have been broken, is all the greater when the glass sheets are not fastened to the frame, for example by adhesive bonding, but simply set, at least in the lateral parts, in the jambs of the frame of the aperture closed off by the said glazing. Such an arrangement actually allows the glazing to be removed from its position in the event of deformation of the glazing, the latter not being retained along its lateral parts.
According to another embodiment, the glazing is curved in at least one direction.
According to a preferred embodiment of the invention, the glass sheets have a thickness of between 1 and 3 mm. For smaller thicknesses, it seems that the effect of the pieces of glass bearing against each other when the glass sheet is broken only occurs under certain fracture conditions. Above 3 mm, the laminated glazing according to the invention is satisfactory, but in the case of glazing intended for motor vehicles, it is then necessary to modify the door frames into which the glazing is fitted, because of the thickness, since at the present time the said frames are designed for non-laminated glazing.
The invention also provides a method of reinforcing the tensile strength of an interlayer consisting of at least two sheets of PVB intended to equip laminated glazing in which a material, such as a polyester film, is inserted between the two sheets of PVB.